Scanning electron microscope

ABSTRACT

A scanning electron microscope having a scintillating element of an alkali-free glass doped with a luminescent cerium oxide.

United States Patent [191 Gee et al. Oct. 15, 1974 [54] SCANNING ELECTRON MICROSCOPE 3,472,997 lO/ 1969 Kareh 250/310 [75] Inventors: Alan E. Gee, Needham; Elias Snitzer, Wellesley, both of Mass. Primary Examiner-Archie R. Borchelt [73] Asslgneez Amerlca n Optical Corporatlon, Assismm Examiner c 5 Church Southbndge, Mass- Attorney, Agent, or Firm-H. R. Berkenstock, Jr.; 22 Filed: Apr. 24, 1973 wllllam Nealon [2!] Appl. No.: 354,039

[52] US. Cl. 250/310, 250/483 57 ABSTRACT [51] Int. Cl. H0lj 37/26 [58] Field of Search 250/309, 310, 311, 483,

250567, 361 A scanning electron microscope havmg a scintillating element of an alkali-free glass doped with a lumines- [56] References Cited cent cenum oxide- UNITED STATES PATENTS 3,052,637 9/1962 Bishay 250/483 8 Claims, 2 Drawing Figures I2 I P 20 z: I :l

l 24 la i \,/22 I 36 I 38 1/ i so SHEET 20F 2 PAT ENTEU um I 51914 1 SCANNING ELECTRON MICROSCOPE BACKGROUND OF THE INVENTION This invention relates generally to improvements in scanning microscopes and more particularly to a scanning electron microscope having a glass scintillating element capable of providing substantially longer service life than previous scintillating elements and giving a fluorescence of high intensity and short decay time when bombarded with electrons. The scanning electron microscope including such a glass scintillator may be either of the thermionic (hot cathode) type or the field emission (cold cathode) type.

Present-day scanning electron microscope systems include an electron gun (either cold or hot cathodes) providing a source of electrons which are accelerated and focused by anode means disposed downstream from the electron source. These anodes form the supply of electrons into a beam which is directed to bombard a specimen disposed further downstream from the source than the accelerating and focusing anodes. The bombarded specimen, under the influence of the high energy electrons striking its surface, emits secondary electrons, the number of which carry information of the physical character of the specimen being bombarded. These secondary electrons are accelerated and directed to bombard a scintillation device. The scintillator produces a light upon bombardment by the electrons, the intensity of which varies according to the number of the electronsstriking the scintillator. This luminescence of the scintillator is transmitted to a photo-multiplier or other photon detecting device which, in turn, generates an electrical signal approximately proportional to the light signal sensed by it. The electrical signal output of the photo-multiplier may then be displayed as on a cathode ray tube to reveal the information carried by the emitted secondary electrons. I

in scanning electron microscopes, the electron beam is caused to scan the specimen in a longitudinal and lateral pattern similar to conventional television picture generation and a television-type monitor is used to display the signal of the photo-multiplier. In so doing, the signal of the photo-multiplier is utilized as a brightness control for the cathode ray of the television-type monitor and the sweep of that monitor is synchronized to the sweep of the electron beam across the specimen. Thus, the picture displayed on the monitor provides a physical representation of the secondary electron emission across the scanned surface of the specimen.

Thermionic electron microscopes have been known for many years and have become highly developed. With the advent of the scanning mode of operation in thermionic electron microscopes, a new dimension in specimen examination was achieved. The electron beam could be traced across the surface of the specimen and the secondary emission detected and recorded to give a detailed indication of the structure of the specimen at its surface. Certain problems were encountered in providing a usable output from such devices. The beam first had to be concentrated into a finely focused spot on the face of the specimen and then caused to track across its surface in the usual longitudinal and lateral pattern. In the high focusing or demagnification of the electron beam into a small spot the beam current may be reduced to a very low level causing the population of the secondary emission to be somewhat less than desired. However, the encountered problems were technical ones which were overcome by diligent effort in view of the significant advantages gained by the scanning electron microscope and, principally, the more detailed information available regarding the surface character of the specimen.

Recently, a second type of scanning type of electron microscope has become commercially available. This type includes a cold cathode or a field emission tip as the electron source. This high brightness field emission tip has proved to be a much more productive source of electrons, such that beam currents as measured at the focused spot on the specimen can be as much as 1,000 times greater than those experienced with conventional scanning electron microscopes (10 amps as compared to 10 amps in thermionic devices). The increased beam current at the specimen has greatly-increased the secondary electron emission and enabled a much faster scanning mode of operation which further contributes to greater contrast in the viewed secondary emission pattern as displayed on monitored devices. However, the many fold increase in beam and secondary emission currents has disclosed weaknesses in the scintillating devices previously used.

Scintillating devices 'used in thermionic scanning electron microscopes and in the early days of field emission electron microscopes are commonly composed of organic fluorescing material, either coated on or dissolved in plastics (such as plexiglas). A common source of scintillating devices for such machines is the Pilot Chemicals, Inc. of Watertown, Massgand a common material being the Pilot B scintillator material. These common plastics may be further coated or brushed with fluorescing material. Prior to the entry of the field emission type of scanning electron microscope, the low current density of electron bombardment caused no problematical rapid deterioration of the scintillating elements. Previous experience with scintillating devices in nuclear radiation experiments also involved extremely low current densities such that the susceptibility of the conventional scintillating materials to degradation by the currents was not considered a significant problem. The organic fluorescing materials chosen for electron detection such as the Pilot B material provided extremely good fluorescence under the bombardment currents experienced as well as having short decay times such that the light output of the device is quite closely proportional to the instantaneous electron bombardment.

Other scintillating materials were known. However, many of these were similarly rejected as scintillating elements in the field emission type of scanning electron microscopes since their fluorescent decay times were long by comparison (over 100 nano seconds for a percent decay, for example). In the field emission type of scanning electron microscope, a fast scan is used requiring the fluorescence of the scintillator to have a significantly shorter decay time.

When the conventional scintillating devices such as the organic fluorescers were utilized in the field emission scanning devices having high beam currents (typically l nano amp as compared to l pico amp for the thermionic device, for example) the scintillating devices showed significant deterioration after a service life of only a few hours. Naturally, a requirement of changing'the scintillating device as often as once a day or perhaps several times a week was highly undesirable.

We have discovered that certain scintillating materials exist which give the desired response for electron bombardment, the use of which in combination with electron microscopes was not previously known. We have found that materials such as silicate glass doped with a rare earth metal oxide when ground, polished and mounted with a suitable light pipe in the specimen chamber in the scanning electron microscope perform remarkably well. While some glass materials were known and used as scintillators, their application was, insofar as we can determine, more exclusively in the application area wherein the materials were bombarded with ultra-violet radiation as compared to electron beam radiation. Additionally, other scintillating glasses have been used as neutron detectors in nuclear radiation experiments. g

The known glass scintillators were generally alkali silicate glass including in particular the alkali constituent'lithium. We have found that the presence of the alkalis contributes to the degradation of the lighttransmitting properties of such glass when bombarded by electrons at beam currents of the order used in field emission scanning electron microscopes. This problem is generally identified as solarization and results in the glass taking on a darkening or browning.

As previously mentioned, a solution to the problem was complicated by the fluorescing light intensity and decay requirements of the scintillator for the scanning electron microscope.

' Some scintillating glasses were known which were alkali-free (see,.for example US. Pat. Nos. 3,527,711 and 3,634,711). However, these were developed primarily for uses in conjunction with lasers. The significant fluorescence of these (principally rare earth doped) was responsive to ultraviolet radiation. Some 'cathodo-luminescence was reported in the rare earth doped material; however the enumerated uses (decorative, signs, cathode ray tube screens) suggest prolonged phosphorescence, and not the required quick delay fluorescence for usage in field emission type scanning electron microscopes. Further, visual examination of the pure? silicate glasses revealed inordinate numbers of dis continuities', inclusions, flaws, etc., all of which significantly detract from the optical-transmitting capability. While the optical quality of these glasses might be improved some, the poor quality of the observed samples does not suggest that anything approaching true optical'quality could be achieved. Physical examination of these materials tends to convince one that they would be unacceptable as a scintillating device for electron detection in the electron microscope application. The pure silicate glasses are so difficult to produce and so difficult to dope with luminescent-promoting materials, such as rare earth oxides, that their resulting structure is full of the previously-mentioned inclusions, discontinuities, flaws, etc. and their resulting poor optical properties suggest unacceptability as a light generator and transmitter. However, when the tri-valent cerium doped material is formed into the shape of a conventional scanning electron microscope scintillator (a hemisphere) and disposed on the end of a glass light pipe (which also is compatible with the refractive index of the glass scintillator) a long life responsive scintillating element for scanning electron microscopy is produced.

I SUMMARY OF THE INVENTION I The present invention is directed to the combination of a scanning electron microscope and a glass scintillator. In accordance with certain features of the invention, an alkali-free glass scintillating device, having a cerium oxide dopant is disposed on a glass light pipe with an optical quality adhesiveand coupled with a photo-multiplier device is incorporated in the specimen chamber of a scanning electron microscope.

BRIEF DESCRIPTION OF THE DRAWINGS i DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, reference numeral 10 indicates a scanning electron microscope which embodies the present invention. The illustrated microscope 10 is of the field emission type wherein a cold field emission tip 12 disposed in a gun chamber 14 maintained in ultra-high vacuum (in the order of 10 Torr.) serves as the source of electrons. US. Pat. No. 3,678,333 is illustrative of a field emission tip type of scanning electron microscope. The illustration of the present invention in combination with the-field emission type microscope, is by way of illustration and not by way of limitation, for the invention might be embodied equally wellin a scanning electron microscope having a thermionic electron source. These later devices include a hot cathode generally in the form of a hairpin-shaped tungsten filament disposed within the influence of a grid usually called a Wehnelt cylinder.

Referring again to the field emission type device illustrated in FIG. 1, a first anode 16 of a focusing electrode 18 is formed in an annular shape with a central opening 20. Spaced downstream from the first anode 16 is a second' anode 22 (afocusing electrode) having a centralmore positive with respect to tip 12 than the voltage applied to first anode 16. The beam 26 is thus influenced by anode 22 to further accelerate the electrons of beam 26 focusing them as well, to subsequently cause them to impinge upon the surface of a specimen S disposed in specimen chamber 30.

Chamber 30 is a vacuum holding chamber, similar to gun chamber 14, however operable at less vacuum (in the order of 10' Torr.) Chamber 30 is isolated from gun chamber 14 by means of walls 32 and includes a central aperture 34. Valving means for sealing aperture 34 to facilitate changing of specimen S is described in the aforementioned US Pat. No. 3,678,333.

Beam 26, after leaving anode 22 and prior to reaching specimen S, passes between deflection means 36 and laterally of its surface in the familiar rectangular scan pattern.

When the surface of specimen S is bombarded by the primary electrons of beam 26, secondary electrons are emitted from the specimen at the bombarded point, the amount of secondary electrons being variable and generally depending upon the material of the particular bombarded point in the incident angle of the electron beam 26. Additionally, primary electrons of beam 26 may be reflected from the surface of specimen S and co-mingled with the secondary electrons, these reflected primary electrons being known as back scattered electrons. The secondary electrons emitted from specimen S are usually of substantially lower energy level than the primary electrons rendering them discernable from the back-scattered electrons. The number and energy. of the secondary emissions contains information as to the character of the surface of specimen S. These secondary electrons are detected by detecting means 40 including a scintillator 42 and a photomultiplier 44. Scintillator 42 is a device which is responsive to bombardment of corpuscular radiation (such asby positive or negative ions or electrons) providing a luminescence in response thereto. Scintillator 42 includes a fluorescent tip 46 fixedly secured to a light pipe 48 which, in turn, is operably connected to photo-multiplier 44. When the secondary electrons bombard scintillator 42 at tip 46, the tip luminesces with a light signal whose amplitude is generally proportional to the number and energy of electrons bombarding the tip. The luminescence is transmitted from the tip through light tube 48 to a photo-multiplier 44 which, in turn, generatesan electrical signal proportional to the luminescence of tip 46. This electrical signal of photo-multiplier 44 is then supplied through amplifying means 50 to a display such as television monitor 52. Scanning control 38 is also operably connected to television monitor 52 to synchronize the monitor to the scan of electron beam 36 so that the observed secondary emission of specimen S can be displayed across the face of the television monitor 52.

Referring now to FIG. 2, an enlarged view of the scintillating device 42 for utilization in a scanning electron microscope is shown. A mounting plate 60 adapted to be disposed on specimen chamber 30 is fitted with suitable sealing means such as the illustrated O-rings 62 for maintaining the previously-mentioned vacuum in said specimen chamber. Generally centrally disposed in mounting plate 60 is an aperture 64 adapted to receive light pipe 48. As with mounting plate 60, suitable sealing means 62 are disposed within the aperture 64 to coact with light pipe 48 to maintain the desired vacuum in chamber 30. Disposed internally of chamber 30 at the end of light pipe 48 is glass scintillating tip 46. It is fixedly secured on the end of pipe 48 with a transparent adhesive having properties of good light conduction,

resistance to softening or other heat degradation (to at least 200C), and capable of an intimate bond between tip 46 and pipe 48. In the illustrated embodiment, an adhesive known as Optical Coupling Epoxy available from Nuclear Enterprises, Inc. of San Carlos, Calif, is

used. It is advantageous to bake out the specimen chamber with the scintillator mounted therein and the resistance of the adhesive to heat permits such a practice. An alternative to an adhesive coupling for tip 46 to pipe 48 is a mechanical coupling, suchas aclip or the like. These mechanical couplings, however, do not ensure a positive contact across the interfacing surfaces and thus permit loss or attenuation of the transmitted light signal;

Light pipe 48 is constructed of a glass material of compatible transmission characteristics to the host glass of tip 46 (e.g., of similar index of refraction) to insure maximum transmission of light across the interface. In the illustrated embodiment, core glass commercially available under the name Schott BaF Type 4 has been found satisfactory. The glass pipe in the preferred embodiment is similar to a light-conducting glass fiber however having dimensions substantially greater. In the illustrated embodiment, the pipe has a diameter of approximately three-eighths inches and is of the clad type wherein an outer reflecting layer of substantially lower refractive index glass is disposed over a lightconducting core. In the illustrated embodiment, the cladding Kimble Type EN-l, has been found satisfactory.

Tip 46, in one embodiment, is'constructed of a high purity silicate glass doped with a cerium oxide with the active fluorescing ion being Ce. Thematerial for the tip may be generally prepared (doped) with'the chosen cerium material according to the methods disclosed in the HS. Pat. No. 3,527,71 1. Tip 46 is generally ground and polished to a hemispherical shape having a radius of about three-sixteenths inches and disposed on the end of the light pipe having a similar radius.

Physical requirements of the glass are'that it be resistant to degradation due to bombardment of electrons (solarization) and be able to withstand high vacuum (about 10" Torr.) and heat (about 200C). Further, the glass must be able to be doped with cerium oxide to a sufficient extent to efficiently fluoresce when bombarded with electrons. For use in fast scanning electron microscopes, it has been found that the fluorescence of the scintillator must be of comparatively short duration (less than 100 nano seconds) to provide a resolution compatible with the characteristics of the fast scan of the field emission type of scanning electron microscope. Thus, the host and dopant-must exhibit a combined influence upon each other to present the aboverequired characteristics. It has been found that a commonelement to solarization inscintillating glasses is the alkali which enhances the workability of the glass. The mentioned high purity silicate glasses, being alkalifree, have been found to exhibit the required characteristics in combination with the scanning electron microscope.

To further ensure transmission of the light generated in scintillator 46 through pipe 48 tophoto-multiplier 44, the base of tip 46 is precision ground and polished as in the receiving end of pipe 48. Such additional procedure enhances the positive interface across tip 46 and pipe 48, which, in turn, adds to the uniformity of transmission of light across the interface as well as minimizing the requirement of adhesive to bond the two together. 7 r ln its installed condition, scintillator 42 is provided with a grid cap 66 disposed generally aroundthe end of light pipe 48 containing tip 46. Grid cap 66 is connected to biasing means 68 through grid bias lead 70 extending from cap 66 through mounting plate 60 to bias means 68. Similarly, tip 46 is connected to biasing means 72 by means of a scintillator bias lead 74 extending generally from the base of tip 46 through mounting plate 60 and insulator 73 to bias means 72.

In operatioma grid voltage in the order of l volts to +300 volts may be applied to grid cap 66. This voltage may either reject or accelerate the emitted secondary electrons from the surface of specimen S toward scintillator 42. In the instance when the secondary electrons are rejected, the scintillator may detect the backscattered electrons. The bias voltage in the order of 10 volts is applied to tip 46 to maximize the capture 0f electrons on the hemispherical tip at its apex.

As previously discussed, the striking of the secondary electrons on the scintillator tip 46 generates a luminescence proportional to the number of electrons striking the tip which is conveyed by the light pipe 48 to photomultiplier tube 44. In the initial stages of photomultiplier tube 44, the light signal is converted to an electrical signal proportional to the varying intensity of the light generated by the secondary electrons. This electrical signal is conditioned (amplified, etc.) to serve as brightness input of the cathode ray within television monitor 52. Thus, as the electron beam 26 and the cathode ray of monitor 52 are simultaneously scanned over the specimen and tube, respectively, a secondary electron image of the specimen S can be observed on the fluorescent screen of the television monitor.

The incorporation of the all-glass scintillator/light pipe into the scanning electron microscope provides a combination of response and endurance not previously enjoyed or contemplated. The combination also offers a flexibility not previously enjoyed since it may also serve as the detector for a field emission scanning electron microscope operating as an ion probe, as discussed in US. Pat. No. 3,678,333 and application for Letters Pat. Ser. No. 225,970 both commonly assigned to the assignee of the present invention. Likewise, though the alkali-free glass chosen in the preferred embodiment is a silicate glass, it is noted other glasses, doped with a luminescent agent might also be used, so long as exhibiting the previouslyelaborated characteristics. Thus, it is evident that certain modifications and variations of the invention may be made without departing from the spirit and scope of the appended claims.

' We claim:

1. In combination with a scanning electron microscope wherein a specimen is bombarded with a beam of corpuscular particles, having a charged particle generator, an objective particle lens for focusing said beam from said generator onto a selected area of said specimen and beam-deflecting means for causing said beam to scan said specimen in a predetermined manner, secondary emission detecting means having a scintillating element consisting of an alkali-free glass doped with a luminescent cerium oxide fixedly secured to glass lighttransmitting means, and a photo-electric element for converting said light generated by said scintillating element into an electrical signal proportional to said light.

2. The combination according to claim 1 wherein said cerium oxide is in the trivalent state.

3. The combination according to claim 2 wherein the concentration of said cerium oxide in said glass is at least 10 parts per million.

4. The combination according to claim 2 wherein said concentration of cerium oxide is at least 50 parts per million.

5. The combination according to claim 1 wherein said glass light-transmitting means is a glass-light conducting core clad with a glass shell having a refractive index lower than the refractive index of'said core.

6. The combination according to claim 1 wherein said scintillating element is secured to'said glass lightpure grade. 

1. In combination with a scanning electron microscope wherein a specimen is bombarded with a beam of corpuscular particles, having a charged particle generator, an objective particle lens for focusing said beam from said generator onto a selected area of said specimen and beam-deflecting means for causing said beam to scan said specimen in a predetermined manner, secondary emission detecting means having a scintillating element consisting of an alkali-free glass doped with a luminescent cerium oxide fixedly secured to glass light-transmitting means, and a photo-electric element for converting said light generated by said scintillating element into an electrical signal proportional to said light.
 2. The combination according to claim 1 wherein said cerium oxide is in the trivalent state.
 3. The combination according to claim 2 wherein the concentration of said cerium oxide in said glass is at least 10 parts per million.
 4. The combination according to claim 2 wherein said concentration of cerium oxide is at least 50 parts per million.
 5. The combination according to claim 1 wherein said glass light-transmitting means is a glass-light conducting core clad with a glass shell having a refractive index lower than the refractive index of said core.
 6. The combination according to claim 1 wherein said scintillating element is secured to said glass light-transmitting means with an optical quality transparent adhesive.
 7. The combination according to claim 6 wherein said adhesive is non-heat softenable to a temperature of at least 200*C.
 8. The combination according to claim 1 wherein said alkali-free glass is a silicate glass of commercially pure grade. 